The present invention relates generally to variable displacement pumps, and more particularly to variable displacement pumps for supplying working fluid to a hydraulic device mounted on a vehicle, for example, to an automotive hydraulic power steering system.
Japanese Patent Application Publication No. 2004-218430 discloses a variable displacement pump for a hydraulic power steering system mounted on an automotive vehicle. The variable displacement pump includes: a body; a rotor mounted in the body, and arranged to be rotated by a driving source; and a cam ring mounted radially outside of the rotor in the body, and arranged to move with a change in an eccentricity of the cam ring with respect to the rotor. Change of the eccentricity causes a change in a specific discharge rate as a quantity of discharge of working fluid per one rotation of the rotor. The variable displacement pump further includes an electromagnetic valve arranged to actuate the cam ring for regulating the eccentricity. The electromagnetic valve is controlled to change a pump discharge rate as a quantity of discharge of working fluid per unit time, with reference to a state of operation of the vehicle.